A conserved BAH module within mammalian BAHD1 connects H3K27me3 to Polycomb gene silencing

Trimethylation of histone H3 lysine 27 (H3K27me3) is important for gene silencing, (epi)genome organization and organismal development. In a prevalent model, the functional “readout” of H3K27me3 in mammalian cells is achieved through the H3K27me3-recognizing chromodomain harbored within the chromobox (CBX) component of canonical Polycomb repressive complex 1 (cPRC1), which induces chromatin compaction and gene repression. Here, we report that binding of H3K27me3 by a Bromo Adjacent Homology (BAH) domain harbored within BAH domain-containing protein 1 (BAHD1) is required for overall BAHD1 targeting to chromatin and for optimal repression of the H3K27me3-demarcated genes in mammalian cells. Disruption of direct interaction between BAHD1-BAH and H3K27me3 by point mutagenesis leads to chromatin remodeling, notably, increased histone acetylation, at its Polycomb gene targets. Mice carrying an H3K27me3-interaction-defective mutation of Bahd1BAH causes marked embryonic lethality, showing a requirement of this pathway for normal development. Altogether, this work demonstrates an H3K27me3-initiated signaling cascade that operates through a conserved BAH “reader” module within BAHD1 in mammals. Overall design: To dissect the role for an H3K27me3-binding BAH module within Bahd1 in gene expression regulation, we carried out transcriptome profiling of the mouse embryonic fibroblast (MEF) cells that carry either WT Bahd1 allele or an engineered W659G homozygous mutation within Bahd1 BAH. W659G disrupts the binding of the Bahd1 BAH motif to H3K27me3.

组蛋白H3赖氨酸27三甲基化（H3K27me3）在基因沉默、（表观）基因组组织以及个体发育过程中发挥关键作用。
在主流模型中，哺乳动物细胞内H3K27me3的功能性“读出”是通过经典多梳抑制复合体1（canonical Polycomb repressive complex 1，cPRC1）的染色盒（chromobox，CBX）亚基所携带的H3K27me3识别型染色质结构域（chromodomain）实现的，该结构域可诱导染色质浓缩与基因抑制。
本研究发现，BAH结构域包含蛋白1（BAHD1）所携带的溴邻域同源结构域（Bromo Adjacent Homology，BAH）与H3K27me3的结合，是BAHD1整体靶向染色质，以及哺乳动物细胞中H3K27me3标记基因实现最佳抑制效果所必需的过程。
通过点突变破坏BAHD1-BAH结构域与H3K27me3之间的直接相互作用，会导致其多梳靶基因位点发生染色质重塑，具体表现为组蛋白乙酰化水平显著升高。
携带Bahd1基因BAH区域H3K27me3结合缺陷突变的小鼠会出现严重的胚胎致死表型，表明该信号通路对于个体正常发育不可或缺。
综上，本研究证实了哺乳动物中一条由H3K27me3启动的信号级联反应，该通路通过BAHD1内保守的BAH“识别模块”发挥功能。
总体实验设计：为解析BAHD1内的H3K27me3结合BAH模块在基因表达调控中的作用，我们对分别携带野生型Bahd1等位基因、或Bahd1 BAH区域内工程化构建的纯合W659G突变的小鼠胚胎成纤维细胞（mouse embryonic fibroblast，MEF）进行了转录组测序分析。W659G突变可破坏BAHD1 BAH基序与H3K27me3的结合能力。